During the operation of steam as well as gas turbines, in some situations, such as during the start-up of a turbine, when the rotor is overcoming its natural frequencies and its vibrations are, as a rule, at the highest level, undesirable rubbing between the stator and the rotor may occur (“rotor-stator rub” or “rubbing”). In the first phase of this rubbing it is especially the seals arranged between the rotating and statical parts of the turbine that are abraded, and, as a result, the amount of leaking medium increases and the turbine efficiency decreases. In cases when rotor-stator rubbing is not detected in time and is not eliminated by an appropriate intervention of the machine operator, such as by changing the running speed during the start-up or run-down, or by changing parameters of the jacking oil during the operation on a turning gear, both the rotor and the stator may be heavily damaged, or an breakdown of the whole turbine leading to considerable economic damage may occur.
The rotor-stator rubbing may be either partial, when the rubbing between the rotor and the stator is brief, but at least several times repeated, or it is full rubbing, i.e. a continuous or almost continuous rubbing between the rotor and the stator. Nevertheless, the full rotor-stator rubbing is always preceded, at least for a short time, by the partial rotor-stator rubbing.
At present, detection of the partial rotor-stator rubbing is based especially on offline analysis of vibration signals, when during the measurements the machine operator monitors the overall level of vibrations as well as the phasor of the first harmonic component of the rotational frequency in vibration signals. If a step change occurs in overall vibration, or in rotation of the phasor of the first harmonic component with variable or periodically varying amplitude, rotor-stator rubbing is detected, and after terminating the measurements a detailed data analysis is carried out with the purpose of excluding the possibility of false positive detection. Therefore this approach is not suitable for detecting rotor-stator rubbing during the real operation of the turbine and can only be used for laboratory research or experimental purposes.
Another approach for detecting partial rotor-stator rubbing is based on the fact that apart from the change in the phasor of the first harmonic component of the vibration signals, the rubbing is also accompanied by formation of subharmonic spectral components, whose frequency corresponds to the frequency of impacts of the rotor on the stator. Nevertheless, the disadvantage of detecting rotor-stator rubbing based on monitoring these subharmonic components is the fact that the frequencies of some of them are too close to frequencies indicating other defects, such as the instability of the oil film in the slide bearing, etc., which may result in false positive detection of rotor-stator rubbing, or, on the contrary, in wrong interpretation of expresses of this rubbing as defects of different type.
Furthermore, from patent and non-patent literature there are known also other methods of detecting and/or localizing partial rotor-stator rubbing based on entirely different principles.
For example EP 1533479 discloses a method of detecting and/or localizing rotor-stator rubbing, in which operating characteristics of the turbine are monitored, such as the housing temperature, the rotor excentricity, pressure in the condenser and the generator load. Simultaneously, a corresponding algorithm independently analyzes the change of individual characteristics and if an abnormal change, or a step change in one of the characteristics occurs, this change is interpreted as a side-effect accompanying rotor-stator rubbing. The drawback of this method is that a step change in the monitored characteristic may relate to other phenomena or defects which occur during the operation of the turbine, therefore there is very high probability of false positive detection of rotor-stator rubbing. Considering the dynamics of the monitored characteristics and assumed sensitivity of the algorithm used, it is moreover impossible to regard the analysis of the change of these characteristics and the subsequent detection of rotor-stator rubbing as a sufficiently fast method which could be actually employed during the operation of a turbine.
From US 2008240902 a method of detecting rotor-stator rubbing is known which consists in monitoring the temperature of the turbine stator at least on part of its circumference, wherein possible local increase in temperature is attributed to the friction between the rotor and the stator and is evaluated as the presence of the rotor-stator rubbing. Also, according to the positioning of the temperature sensor which has registered the increase in temperature, it is possible to roughly localize the place of the rubbing on the stator. The shortcoming of this method is that at the moment when, as a result of friction between the rotor and the stator, sufficient amount of heat is produced to be registered by the temperature sensor, a period of time can pass since their first contact—a period that could last in the order of minutes, during which the rotor, stator, or possibly other parts of the turbine may be already seriously damaged, and it is impossible to prevent the stator from bending in any other way than by the machine shutdown. Another disadvantage is that in case of serious rotor-stator rubbing which occurs along the entire circumference of the stator or its larger part, the increase in temperature of the stator is registered substantially uniformly by most or all the temperature sensors located in the stator and due to this it does not have to be always classified as rotor-stator rubbing. Furthermore, a major disadvantage is the fact that additional installation of plurality of temperature sensors in the body of the turbine stator requires the turbine shutdown and relatively large investments.
In addition, US 20090003991 describes a method of detecting rotor-stator rubbing, which consists in continuous measurements of the distance of the rotor blades from the stator and/or the height of the rotor blades. Similarly to the preceding case, the drawback of this method is the necessity of additional installation of special sensors in the turbine stator and increased overall costs related to it. Although it is possible to detect rubbing between the blade and the stator by measuring the space between the rotor blade and the blade seal in the stator, real probability of successful and prompt detection of rotor-stator rubbing by means of this method is extremely low due to the fact that clearances between the blade and the stator are larger than those between the shaft and the stator and, therefore, rotor-stator rubbing occurs primarily on the turbine shaft. A further disadvantage is possible false positive detection of rotor-stator rubbing upon the change in the parameter sensed by sensors, which could be also caused by another factor, for instance it could result from rotor eccentricity, change of its rotational speed, etc.
The goal of the invention is to eliminate the disadvantages of the background art and propose a new method of detecting partial rotor-stator rubbing during the operation of a turbine, which would eliminate shortcomings of the prior art and would enable to detect rotor-stator rubbing in real time, as soon as possible after it occurs, and at the same time to localize it as precisely as possible.